1. Field of the Invention
Implementations of various technologies described herein generally relate to seismic data processing, and more particularly, processing seismic data acquired using over/under streamers.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposits. Since many commercially valuable hydrocarbon deposits are located beneath bodies of water, various types of marine seismic surveys have been developed. In a typical marine seismic survey, seismic streamers are towed behind a survey vessel. The seismic streamers may be several thousand meters long and contain a large number of sensors, such as hydrophones, geophones, and associated electronic equipment, which are distributed along the length of the seismic streamer cable. The survey vessel may also include one or more seismic sources, such as air guns and the like.
As the seismic streamers are towed behind the survey vessel, acoustic signals, commonly referred to as “shots,” produced by the one or more seismic sources are directed down through the water into strata beneath the water bottom, where they are reflected from the various subterranean geological formations. Reflected signals are received by the sensors, digitized, and then transmitted to the survey vessel. The digitized signals are referred to as “traces” and are recorded and at least partially processed by a signal processing unit deployed on the survey vessel. The ultimate aim of this process is to build up a representation of the subterranean geological formations beneath the streamers. Analysis of the representation may indicate probable locations of hydrocarbon deposits in the subterranean geological formations.
The seismic streamers may be in an over/under configuration, i.e., one set of streamers being suspended above another set of streamers. Streamers in an over/under configuration may be towed much deeper than streamers in a conventional single configuration.
Conventional techniques for processing seismic data acquired by streamers in a twin over/under configuration generally involve a dephase and sum algorithm and a shift and subtract algorithm. In the application of the dephase and sum algorithm, the data recorded at each streamer may be first processed by a correlation step to dephase the individual surface ghost wavelets. However, this requires explicit knowledge of the surface ghost wavelet, which is typically very difficult to obtain. Hence, the surface ghost wavelet is typically assumed to be that of a flat sea-surface, which renders conventional techniques useless in rough sea conditions.
In the application of the shift and subtract algorithm, the data recorded on one of the twin over/under streamers may be time shifted so that the free-surface ghost energy occurs at the same time on both streamers. Subtraction of one data set from the other may remove this ghost energy. Such shift and subtract algorithms, however, may leave a ghost of the up going wave fields in the processed data. This ghost may not easily be removed due to its spectral notches.